We read with interest the article by Martinelli and colleagues [1] on evaluating risk factors for deep venous thrombosis of the upper extremities. Although the authors claim to have tested their patients for all possible causes of thrombophilia, they did not mention a rare but important cause: factor XII deficiency.

We admitted a 62-year-old white woman who presented with spontaneous deep venous thrombosis in the left axillary vein. Coagulation studies done before heparin therapy was started revealed an abnormal partial thromboplastin time of 114 seconds (the normal range at our laboratory is 25 to 40 seconds). Further work-up revealed that the patient had severe factor XII deficiency. Her factor XII level was 18 U/dL on both one-stage clotting and chromogenic peptide substrate assay. This deficiency had not been identified previously because the patient had never needed a coagulation work-up.